(1) Field of the invention
The present invention relates to a plastic container having frosted appearance and to a process for producing the same. More specifically, the invention relates to a plastic container having frosted shade-off and high-grade appearance which is similar to that of a ground glass container, and to a process for producing the container maintaining good reproducibility.
(2) Description of the Prior Art
Plastic containers have now been widely used for packaging a variety of contents to substitute for glass containers, since they are light in weight and are excellent in shock resistance and other mechanical properties. Particularly, blow-molded containers made of olefin resins such as polyethylene, polypropylene and the like are much demanded since they are relatively cheap in cost and are easily molded.
Here, plastic containers having frosted appearance have been strongly demanded just like the ground glass containers have been widely used. The containers having frosted appearance give an impression of shade off, depth and high grade, and have been demanded particularly in the field of packaging containers for packaging cosmetics, toiletry goods and any other household products.
A technique which has heretofore been adopted to impart matted or frosted appearance to the plastic containers comprises sand-blasting the cavity surfaces of a metal mold that effects the blow-molding, and transferring the rugged pattern formed on the cavity surfaces onto the outer surfaces of the plastic container that is being molded.
In recent years, Japanese Laid-Open Patent Publication No. 86260/1992 is disclosing a plastic container having matted appearance in which at least the outer surface of the plastic container is formed of a blend of an ethylene-propylene block copolymer having an ethylene content of 3 to 10% by weight and a melt flow rate of from 0.8 to 10 g/10 min. and a high-density polyethylene. There is further disclosed that the inner surface of this multi-layer container is formed by an ethylene-propylene random copolymer having an ethylene content of 1 to 8% by weight and a melt flow rate of 1 to 5 g/10 min.
Among these prior techniques, the technique which sand-blasts the cavity surfaces of the metal mold has a defect in that frosted appearance is not reproduced to a satisfactory degree on the container that is molded. This defect is presumably related to a problem that the rugged pattern on the surface of the metal mold is not faithfully reproduced on the surface of the container when the plastic material that is under the molten state is cooled upon contact with the surface of the metal mold and to another problem that even when the rugged pattern on the surface of the metal mold is faithfully reproduced on the surface of the container, the frosted appearance does not develop to a satisfactory degree. In relation to these problems, it should be noted that the glass is completely amorphous while the plastic material is more or less crystalline and the glass is completely transparent while the plastic material has an internal haze.
According to the method of the above Japanese Laid-Open Patent Publication, considerably excellent frosted appearance is obtained as compared with those of when an ordinary olefin resin is used. Compared with the ground glass containers, however, the container is not yet satisfactory with respect to shade off, depth and high-grade appearance, leaving room for improvement.